1. Field of the Invention
The present invention generally relates to mobile communication techniques with radio relay and more particularly relates to a radio relay apparatus and a radio relay method in radio relay systems.
2. Description of the Related Art
In the mobile communication techniques, it is important to achieve the widest coverage areas possible. To this end, it is discussed that radio relay techniques are to be utilized in next generation radio communication systems. In the radio relay techniques, for example, in downlinks, signals transmitted from a base station are received at a relay station, and the received signals are amplified at the relay station and retransmitted to a mobile station. In this manner, the mobile station can receive the signals at increased power, resulting in wider coverage.
Moreover, it is also important to improve frequency utilization efficiency. To this end, it is discussed that a MIMO (Multiple-Input Multiple-Output) technique is to be applied to the above radio relay techniques. A MIMO applied radio relay system is disclosed in “Linear precoder and decoder for two-way AF MIMO relaying system” by N. Lee, H. Park and J. Chun (Proc. of VTC-2008 Spring, May 2008). In this radio relay system, weight matrices for reducing inter-antenna interference and inter-user interference are optimized to minimize a bit error rate.
In downlinks, a relay station receives signals from a base station and transmits the signals to a mobile station. In uplinks, on the other hand, the relay station receives signals from the mobile station and transmits the signals to the base station. Here, if the signal transmission and the signal reception are performed simultaneously regardless of the uplinks and downlinks, the signals transmitted from the relay station would be received at that relay station, which may cause circuit oscillation. This phenomenon is called loop interference or self-interference, and if the loop interference is strong, the relay station would fail to relay the signals properly. In WO 2008/004916, this problem is addressed by estimating such a loop channel, calculating a feedback type of weight matrix (canceling matrix) for removing the loop interference, and reducing the inter-antenna interference and the inter-user interference while removing the loop interference. WO 2008/004916 attempts to reduce the inter-antenna interference and the inter-user interference by determining a feedforward type of weight matrix (precoding/weighting matrix) in accordance with a ZF (Zero-Forcing) method, for example.